1. Field of the Invention
The present invention relates to a method for implanting hydrogen ions to a predetermined depth of the overall body of a semiconducting substrate, such as silicon (Si), an insulating substrate made of SiC, glass, or plastic, or a metal substrate. This invention further relates to an implanting apparatus.
2. Background of the Invention
Purposes for implanting hydrogen ions into a substrate fall into two categories. One of these types is directed to forming a weak porous layer (a void layer) in the substrate by implanting hydrogen ions to shear the substrate. The other type is directed to improving the physical properties of a required object. These various purposes for performing of implantation of hydrogen will now be described in greater detail.
1. Implantation of Hydrogen Ions for the Purpose of Manufacturing SOI Substrate
A Silicon On Insulator (SOI) substrate is a substrate having single crystal Si on an insulating layer thereof. A portion of the SOI substrate has a thick insulating substrate on which Si is placed (a Si/insulating substrate). For example, a structure is known in which a thin Si film is formed on sapphire. However, hetero growth on different crystal suffers from frequent crystal defects. No cleavage can be used and, thus, the cost cannot be reduced. Therefore, a major portion of SOI substrates have a triple-layer structure, the overall body of which is made of Si, and in which a thin insulating layer and single crystal Si exist (Si/insulating layer/Si substrate). The insulating layer is made Of SiO2. That is, the triple-layer structure is Si/SiO2/Si substrate.
The Si wafer is a low-cost wafer and those having a high quality can easily be obtained. Since the SOI substrate has a structure in which Si is formed on Si, the lattice constant is the same and the number of defects is small. Since cleavage exists, separation of the device can easily be performed. To manufacture the foregoing substrate, an inner porous layer is formed by implanting hydrogen ions. Then, another Si wafer is bonded to perform shearing at the porous layer, and then the surface is polished so that the SOI is manufactured. The foregoing method will be described in greater detail below.
2. Implantation of Hydrogen Ions to Manufacture Single Crystal Si/Glass Substrate
A substrate for a liquid crystal device incorporates a multiplicity of thin-film transistors manufactured on amorphous silicon (a-Si) on a glass plate. Although the foregoing structure is a main portion of the foregoing substrate, the mobility of carriers of a-Si is too low to realize a high speed operation. At present, substrates for liquid crystal devices having the highest performance incorporate a thin polycrystal silicon film (p-Si) formed on a glass substrate. Since mobility of electrons is superior to that of a-Si, high speed operation is realized. The foregoing structure has been suggested in, for example, Technical Report of Sharp Corporation xe2x80x9cLow-Temperature Polysilicon TFT-LCDxe2x80x9d, Vol. 69, pp. 64 (1997), written by Takashi Itoga, Masataka Ito and Hiroshi Takato.
However, a satisfactory result has not been realized from the foregoing technique. The polycrystal has a multiplicity of grain boundaries, causing scattering of electrons to occur frequently. As compared with single crystal Si, the mobility of electrons is unsatisfactory. Since a multiplicity of grain boundary levels exist in the grain boundary of the polysilicon, electrons are scattered. Therefore, an attempt has been made to decrease the grain boundary levels by implanting hydrogen ions. For example, a suggestion has been made in Japanese Patent Laid-Open No. 8-97432 entitled xe2x80x9cMethod of Manufacturing Thin-Film Semiconductor Apparatusxe2x80x9d filed by Nobuaki Suzuji. According to the foregoing disclosure, when annealing is performed by implanting hydrogen ions, hydrogen terminates Si in the grain boundaries. Thus, the levels can be decreased and the mobility is, therefore, raised.
The polycrystal thin Si film, however, has another problem as well as the low mobility. Since electric currents easily flow along the grain boundary of the polycrystal Si, a great leak current flows between the source and the drain. Therefore, a complicated LDD structure is required. As a result, the SOG (System On Glass) has no possibility of realization. The SOI is used such that hydrogen is implanted into Si to form a porous layer so as to be bonded to a glass plate. Then, a Si substrate is sheared from the porous layer to bond the single crystal thin Si film to the glass substrate. Since the substrate is made of glass in place of Si, a void cutting method can be employed which is similar to the method for manufacturing SOI. Therefore, a method may also be employed in which hydrogen is implanted into a Si wafer to form a weak layer so as to be bonded to a glass plate. Then, the Si layer is thinly separated so that the single crystal Si/oxide/glass layer structure is manufactured.
3. Modification of Solar Cell
At present, solar cells using silicon for electric power mainly use monocrystalline silicon, polycrystalline silicon, amorphous silicon or the like. The amorphous silicon is cheap but its photoelectric conversion efficiency is low (about 8%). On the other hand, the photoelectric conversion efficiency of the monocrystalline and polycrystalline silicon can be 15 to 20%. Accordingly, the latter is mainly used.
The monocrystalline or polycrystalline silicon solar cell is cut similarly to a semiconductor substrate. Accordingly, the thickness of the monocrystalline or polycrystalline silicon solar cell should be 500 xcexcm to 600 xcexcm per one sheet. Most of the sheet is wasteful. In order to obtain the photoelectric conversion efficiency of 15 to 20%, it is sufficient that the thickness is several xcexcm to 20 xcexcm. Therefore, the void cut method by hydrogen ion implantation is used. The following two methods are generally used for this purpose:
(1) Hydrogen atoms are implanted to the depth of several xcexcm by the acceleration energy of several hundred KeV to several MeV to perform the void cut.
(2) Hydrogen atoms are implanted to the depth of several tens nm to several xcexcm to perform the void cut. The insufficient film thickness is made up by expitaxial growing before or after the void cut.
4. Implanting Hydrogen Ions into SiC
A method has been suggested with which a similar void cutting method is employed to manufacture a thin SiC film. The SiC is a semiconductor capable of resisting high temperature and permitted to be used for another purpose. A suggestion has been made about a method of manufacturing a thin SiC film by employing a method similar to that for manufacturing the SOI by forming a porous layer into which hydrogen ions have been implanted and by performing delamination. See xe2x80x9cThin-Film Delamination by Implanting H+ and Application of Thin-film delamination to SiCxe2x80x9d, previous thesis for associated lectures of 45-th relative association, 29a-K-2, pp. 803 (1998). However, a substrate having satisfactory qualities has not been manufactured yet. While a variety of attempts have been made, no device has been realized.
As known, the Si-On-Insulator substrate (a so-called SOI substrate) incorporating a single crystal Si semiconductor layer formed on an insulating material has a variety of advantages, for example, high density integration and capability of manufacturing a high-speed device as compared with a usual bulk Si substrate. Therefore, much research and development has been carried out in a multiplicity of facilities. The foregoing advantages have been disclosed in, for example, Special Issue: xe2x80x9cSingle-crystal silicon on non-single-crystal insulatorsxe2x80x9d; edited by G. W. Cullen, Journal of Crystal Growth, vol. 63, No. 3, pp. 429-590 (1983), which notes that two methods may be available to manufacture the SOI substrate. One is a method (SIMOX) for forming an oxide silicon layer by directly implanting oxygen ions. Another method is a bonding manufacturing method called the void cutting method or a smart cutting method by performing implantation of hydrogen ions. Since the present invention relates to a method for implanting hydrogen ions into the wafer, the smart cutting method can be improved.
A method of manufacturing the SOI substrate by the smart cutting method has been disclosed in, for example, xe2x80x9cSmart-Cut: A new silicon on insulator material technology based on hydrogen implantation and wafer bondingxe2x80x9d; Jpn. J Appl. Phys Vol. 36 (1997) pp. 1636-1641. A multiplicity of other documents have been issued. The methods will briefly be described below. The surface of a first Si substrate is oxidized so that a SiO2 film is formed. Then, hydrogen ions with energies of about 100 keV are implanted in a density of 1xc3x971014/cm2 or greater. Thus, a porous layer having great porosity is formed at a depth of about 0.2 xcexcm to about 0.5 xcexcm. Heat treatment is performed so that damage to the surface of the Si layer caused by the implantation is recovered. Then, the first Si substrate is bonded. The insulating layer may be provided with a second Si wafer. Then, shearing force is imparted in the vertical direction so that the first substrate is cut at the porous layer. The surface is then polished so that the SOI substrate is manufactured.
The gas to be injected may be rare gas or nitrogen gas as an alternative to the hydrogen gas. The hydrogen gas is the most preferred gas. The reason for this lies in that hydrogen having a small mass can be implanted to a considerable depth. Moreover, hydrogen does not considerably damage the surface of the Si layer.
The most usual method for implanting hydrogen ions is a method of using an ion implanting apparatus for implanting impurities of B or P. Prior art FIG. 1 shows a method for implanting hydrogen ions by using a representative ion implanting apparatus.
Excitation of plasma is performed by using a thermal filament, microwaves or high frequency. That is, an apparatus using excitation of the filament is employed. A chamber 1, having pressure which can be reduced to a reasonable level of vacuum, is provided with a filament 2. A terminal of the filament 2 leads to the outside through an insulating member 5. A DC filament power source 3 is connected to the two ends of the terminal. The chamber 1 has a gas inlet opening 4 to allow the introduction of hydrogen gas. An arc power source 6 (Vak) is connected between the chamber 1 and the filament 2. An acceleration power source 7 (Vacc) is placed between the negative electrode of the arc power source 6 and a ground. The potential of the chamber 1 is Vacc+Vak.
Three electrodes each of which is provided with a hole are provided on the outside of outlet opening 8 of the chamber 1 so that the holes in the electrodes are aligned. The electrodes are an accelerating electrode 9, a decelerating electrode 10 and a ground electrode 11. The positive electrode of the acceleration power source 7 is connected to the acceleration electrode 9 through a resistor 13. A decelerating power source 12 is connected to the decelerating electrode 10. A quarter circular-arc mass-separating magnet 14 is disposed on an extension of the chamber outlet opening 8 and the electrodes 9, 10 and 11. An ion beam 15 emitted from the chamber 1 is introduced into the mass-separating magnet 14 through the inlet opening 16 so that a curved trace is drawn by the magnetic field. Then, the ion beam 15 is discharged from an outlet opening 17. Since the trace has been adjusted by the mass and energy, H+ ions (which contain only one atom) are allowed to pass through a central trace 26 so as to pass through a slit plate 18. On the other hand, H2+ ions (which, containing two atoms, are more massive) draw an eccentric trace 27, and then collide with the wall of the mass-separating magnet 14 and the slit. Thus, the H2+ ions are eliminated. H+ ions are allowed to pass through the slit plate 18, and then scanned laterally by a scanning mechanism 22 consisting of opposite electrodes 19 and 20 and a variable power source 21. A scanning beam 23 is injected to a Si wafer 24 placed on a suscepter 25.
A plurality of types of positive ions are generated in the hydrogen plasma. If plural types of positive hydrogen ions are implanted, a plurality of hydrogen-implanted layers are undesirably formed. Therefore, only one type of positive hydrogen ions must be selected and implanted in a substrate. To perform the selective implantation, mass separation must be performed. To perform the mass separation, the diameter of the beam must be reduced. That is, a beam considerably thinner than the diameter of the wafer is required. Since the beam has a diameter smaller than that of the wafer, the beam cannot implant ions into the entire surface of the wafer in one operation. Therefore, a scanning mechanism for swinging the beam is required. The presence of the mass separation structure and the scanning mechanism raises a variety of problems.
A method using the ion implanting apparatus to perform mass separation, scanning and implantation of hydrogen ion beam is similar to a conventional impurity-ion implanting apparatus. As can easily be appreciated, such an apparatus is costly and complicated. Since a magnet having a large height must be provided, a great area is required for installation. Since scanning using the beam must be performed, a very long processing time is required for each wafer. Therefore, the throughput is too low to reduce the cost of each SOI substrate. The foregoing fact is a reason why the SOI substrates, although recognized to have advantages, are not widely used.
In recent years, another method has been suggested in which a substrate is exposed to hydrogen plasma and negative pulse voltages are periodically applied to the substrate so as to implant hydrogen ions to the overall surface of the substrate. The foregoing method has been disclosed in xe2x80x9cIon-cut silicon-on-insulating fabrication with plasma immersion ion implantationxe2x80x9d: edited by Xiang Lu S. Sundar Kumar Iyer et. al, Appl. Phys. Lett.71 (19), 1997.
FIG. 9 shows the foregoing technique. Hydrogen gas is supplied into a plasma chamber 200 through a raw-material gas inlet opening 202. Microwaves 204 generated by a magnetron (not shown) and transmitted in a wave-guide pipe 203 are supplied into the plasma chamber 200. A Si wafer 207 is placed on a suscepter 208 in the plasma chamber 200. The suscepter 208 is supported by a shaft 209. The shaft 209 is negatively biased by a negative bias power source 220. The wafer 207 is contacted with plasma 206. When the wafer is negatively biased, positive hydrogen ions H+ and H2+ are implanted to the overall surface of the wafer in one operation.
The foregoing method does not utilize mass separation and therefore the apparatus can be simplified. However, since the mass separating mechanism is not provided, all positive ions (H2+ and H+) in the plasma are undesirably introduced into the wafer. As a result, two porous layers each having great porosity are undesirably formed. In this case, smart cutting of the wafer cannot satisfactorily be performed. Since mass of the molecule (H2) and that of the atom (H) are different from each other by two times, light H+ ions are deeply implanted about two times the depth to which heavy H2+ are implanted when the same accelerated energy is added. A first layer is formed by H2+, while a porous layer which is a second layer is formed by H+.
Cutting at the second layer formed by the H+ ions must be avoided. The reason for this lies in that the first layer is left on the SOI substrate when the SOI substrate is manufactured by bonding another wafer. If separation of the overall surface of the first layer (the porous layer made of H2+) more adjacent to the surface is permitted, no problem arises. If a portion cut at the second layer exists, a surface defect occurs, causing the manufacturing yield to considerably deteriorate.
In the foregoing document, the foregoing problem is overcome by controlling the state of the plasma by optimizing the gas flow rate and supplied electric power such that the ratio of positive ions in the plasma is made to be H2+/H+=90:10. That is, H2+ is implanted at a higher ratio. Since the quantity of the H+ ions is small, relative to the quantity of H+ ions, the thickness of the first layer is reduced. Thus, a contrivance is employed to cause separation to easily occur at the first layer.
However, complete removal of H+ cannot be performed. Therefore, there is a risk that separation at the second layer will occur. The conventional method cannot generate either the H2+ ions or H+ ions in the plasma with priority to a degree that the other ions can be ignored. If a plasma parameter is shifted even in a small quantity, there is a risk that the ratio of the positive ions H2+:H+ will change. In particular, there is a critical risk that the safety of the manufacturing apparatus cannot be ensured.
When H2+ ions are implanted with priority to form the porous layer, the voltage required is about two times the voltage required for H+ to be implanted to the same depth. Therefore, the degree of technical difficulty involved with providing a power source for applying the plasma voltage is raised. Moreover, the cost of such a method is increased. Therefore, some sort of mass separating mechanism is desirable.
A critical problem of the first method is a fact that mass separation is required. In the plasma, the types of positive hydrogen ions include H+ and H2+ ions as described above. If more than one type is implanted, the porous layer is undesirably formed into a multilayered structure. To select only one type of the ion beam, the ion implanting apparatus shown in FIG. 1 must be provided with a mass separating system. Since a large magnet is required, the size and the cost of the apparatus cannot be reduced. Since a thick beam cannot be sputtered, the diameter of the ion beam must be reduced. Because of this reduction in the diameter of the ion beam, ions cannot be implanted to the overall surface of the wafer in one operation. Therefore, a scanning mechanism must be provided to scan the overall surface of the wafer with the beam.
Attempts to solve the foregoing problem by controlling the plasma parameter utilize the method (see FIG. 9) of exposing the substrate to the hydrogen plasma and applying negative plasma voltage to the substrate so as to implant hydrogen ions. However, the foregoing method is still problematic because plural types of positive hydrogen ions are implanted.
It is a first object of the present invention to provide a method and apparatus for implanting hydrogen ions into a semiconductor substrate, an insulating substrate or a metal substrate such that the ions generated from hydrogen are limited to one type.
It is another object of the present invention is to provide an apparatus for implanting hydrogen ions such that the type of generated ions is limited to one type, thereby removing the need to perform the mass separation and reducing the cost and the required installing area.
It is another object of the present invention is to provide an apparatus for implanting hydrogen ions in which scanning is not required because generated ions are limited to one type and thus a high throughput is realized.
As described above, positive hydrogen ions include two types, H+ and H2+. Therefore, only one type of ion cannot easily be generated at a ratio of 80% or higher. If the mass separation is performed, the size and the cost of the apparatus cannot be reduced. What is worse, the throughput is unsatisfactorily low. Therefore, the present invention does not employ the foregoing method.
According to the present invention, a method for implanting negative hydrogen ions comprises the steps of: generating plasma containing hydrogen; generating negative hydrogen ions in the plasma; forming an electric field between the plasma and a substrate; and accelerating negative hydrogen ions from the plasma by using the electric field so as to implant negative hydrogen ions into a predetermined depth of a substrate.
According to the present invention, an apparatus for implanting negative hydrogen ions comprises: hydrogen generating means for generating plasma containing hydrogen; negative hydrogen ions generating means for generating negative hydrogen ions in the plasma; and electric field forming means for forming an electric field between the plasma and a substrate; wherein negative hydrogen ions from the plasma are accelerated by using the electric field so as to implant negative hydrogen ions into a predetermined depth of a substrate.